High-power ultrashort laser pulses have found applications in many fields of modern science. Direct optical amplification of high-power ultrashort pulses results in detrimental nonlinear effects and laser-induced damage of amplifying medium due to extremely high peak power of amplified pulses. In order to mitigate these effects, a technique of chirped pulse amplification (CPA) was developed. Using this technique, ultrashort pulses are stretched by dispersive optical elements before amplification so that the peak power of the pulses in the amplifier is moderate and does not lead to damage. After amplification the pulses are compressed, resulting in high peak power. Pulse compression is performed with dispersive optical elements that are required to have a high laser-induced damage threshold. Traditionally, pulse stretching and compression in chirped pulse amplification systems is performed with a pair of surface diffraction gratings. Conventional technology uses metal-coated gratings. However, a problem with the prior art is that sizes of such stretchers and compressors are large than sizes of amplifiers and the average power of such laser systems is limited by the relatively low damage threshold of the metal-coated gratings.
A breakthrough in development of chirped pulse amplification laser systems was accomplished by the use of chirped fiber gratings which replaced pairs of surface gratings in stretchers and compressors. This approach has dramatically decreased size and increased robustness of chirped pulse amplification systems and enables their use in harsher environment. However, the maximum aperture of fiber chirped gratings does not allow achieving high power laser systems because of the laser induced damage of fibers by the compressed pulse.
The solution to eliminate the limitations caused by small apertures of fiber Bragg gratings was proposed in A. Galvanauskas, M. Fermann, “Optical pulse amplification using chirped Bragg gratings” U.S. Pat. No. 5,499,134 (1996) which is incorporated as a reference, where chirped volume Bragg gratings (or volume diffractive gratings) with dramatically larger apertures (compared to those in fiber Bragg gratings) were proposed for stretching and compression of high power laser pulses. However, no practical ways for making such gratings were described in that patent. The series of practically useful patents with the use of chirped volume Bragg gratings recorded in photo-thermo-refractive (PTR) glass that is described below.
The present invention is related to U.S. Pat. No. 6,586,141 issued on Jul. 1, 2003 to Oleg M. Efimov, Leonid B. Glebov, Larissa N. Glebova, Vadim I. Smirnov and titled “Process for production of high efficiency volume diffractive elements in photo-thermo-refractive glass” which describes a method of fabrication of high efficiency volume diffractive gratings in PTR glass and U.S. Pat. No. 6,673,497 issued on Jan. 6, 2004 to Oleg M. Efimov, Leonid B. Glebov, Vadim I. Smirnov, titled “High efficiency volume diffractive elements in photo-thermo-refractive glass” both having at least one common inventor as the present invention and assigned to the same assignee and which are incorporated herein by reference. The principle of pulse stretching and compression by high-efficiency PTR-glass volume diffractive gratings with variable periods as described in U.S. Pat. No. 7,424,185 issued on Sep. 9, 2008 to Leonid B. Glebov, Emilie Flecher, Vadim I. Smirnov, Almantas Galvanauskas, Kai-Hsiu Liao, titled “Stretching and compression of laser pulses by means of high efficiency volume diffractive gratings with variable periods in photo-thermo-refractive glass” and co-pending U.S. patent application Ser. No. 11/261,077 filed on Oct. 29, 2005 by Emilie Flecher, Leonid B. Glebov, and Vadim I. Smirnov and titled “Spectral and angular filters based on high efficiency diffractive elements with variable period in photo-thermo-refractive glass”.
The first two patents '141 and '497 teach how to make high efficiency uniform PTR-glass volume Bragg gratings working in transmitting and reflecting regimes and how to use them for making different types of optical filters used in lasers and other photonic devices.
The high-efficiency PTR-glass reflecting volume Bragg gratings with spatially variable periods described in the '185 patent have been used to demonstrate stretching of ultrashort pulses of 100 and more femtoseconds in duration to 100s of picoseconds and high-efficiency re-compression to near-transform-limited pulse duration. Due to this period variation in the direction of laser beam propagation, different spectral components of a pulse incident on the grating are reflected by different parts of the grating along this direction. The optical path length differences between the different spectral components leads to a wavelength-dependent group delay. This wavelength-dependent delay forms a temporally and spatially stretched pulse with instant power that is decreased proportionally to a stretching ratio in comparison with that in the original pulse. A stretched pulse being launched to the same or similar grating in opposite direction will be recompressed to its original duration. More detailed description of the principle of pulse stretching and compression by a volume diffractive grating with variable period is given in the '185 patent. The '185 patent enabled commercial fabrication and a wide use of chirped volume Bragg gratings in high power laser systems with chirped pulse amplification.
However, there are several problems that still restrict a further increase of brightness, power and pulse energy in ultrashort laser systems with chirped pulse amplification. The first problem is a limitation of a stretching time resulting from a technologically limited thickness of chirped Bragg gratings. A solution of this problem is proposed in U.S. Pat. No. 7,444,049 issued Oct. 28, 2008 to K. Kim, L. Vaissie, R. G. Waarts, A. Stadler, M. J. Cumbo, titled “Pulse stretcher and compressor including a multi-pass Bragg grating”. This invention proposed multipass propagation of laser beams in chirped volume and fiber Bragg gratings enabled by specially designed beam steering electro-optical components. The proposed approach enables a dramatic increase of stretching time. However, it results in a dramatic complication of stretching and compression devices.
The second problem is spectral phase distortions in stretched and compressed pulses resulting from deviations of stretching time from linear dependence on wavelength (nonlinear chirps) in different types of stretchers and compressors, from different linear and nonlinear dispersion functions in different components of chirped pulse amplification systems (fibers, amplifiers, phase retarders, etc.), and from technological imperfections of available stretchers and compressors (fibers, waveguides, surface gratings and volume gratings). An approach for solving this problem for compressors comprising surface gratings was proposed in A. M. Weiner, J. P. Heritage, and E. M. Kirschner, “High-resolution femtosecond pulse shaping”, J. Opt. Soc. Am. B 5 (8), 1563-1572 (1988). It was demonstrated that the use of a phase mask at the focal plane of a pulse compressor composed with surface gratings separated by two lenses in 4f configuration allows temporal pulse shaping of the ultra-short pulses. However since such configurations are bulky, it is not usable outside a laboratory environment. Moreover, this approach cannot be used for compact and robust pulse stretchers and compressors based on chirped volume Bragg gratings.
The third problem is keeping a normal incidence of incident and diffracted beams in respect to planes of constant refractive index of CBGs. It is important to note that this conventional use of CBG for pulse stretching and compression essentially relies on the fact that incident and reflected beams are parallel to a grating vector of the CBG (i.e. normal to its equal-phase surfaces constituting the grating). It is obvious for anyone skilled in the art that if the reflected beam in this conventional CBG configuration forms a sufficiently large angle θ with respect to the incident beam then the reflected beam will acquire lateral walk off, thus producing an unacceptable distortion of the output beam. Assuming that incident-beam diameter is D, one can calculate that for a single-pass conventional CBG configuration this angle θ has to be smaller than the minimum angle
      θ    min    =      a    ⁢                  ⁢                  tan        ⁡                  (                      D                          2              ⁢                                                          ⁢              L                                )                    .      In other words, conventional CBG configurations requires that θ<θmin. If this condition is not met the reflected beam will be unacceptably distorted.
In order to increase pulse energies from chirped pulse amplification systems by increasing stretched pulse duration beyond what is achievable with a single-pass conventional configuration, it has been proposed in U.S. Pat. No. 7,444,049 to use a multipass configuration for conventional retroreflecting CBGs. This puts even more strict limitation on the reflection angle θ. One can calculate that for N-pass conventional CBG configuration this angle θ has to be even smaller:
      θ    <          θ      min              (        N        )              =      a    ⁢                  ⁢                  tan        ⁡                  (                      D                          2              ⁢                                                          ⁢              LN                                )                    .      
Essential practical limitation of operating at such small angles is that it becomes necessary to use large separation S between any pick-up or folding mirror and CBG. One can calculate that for θ<θ(N)min one needs to have S>L·N. In other words, the separation becomes N-times larger than the thickness L of the CBG itself. This makes such a conventional multipass CBG arrangement very large, in contradiction to the intended technological goal. In the '049 patent this was addressed by proposing complicated beam-switching arrangements, which make the conventional CBG multi-pass arrangement very complicated and difficult to implement in practice. Moreover, this requirement to keep parallel a propagating beam and a grating vector in any point of the CBG implies extremely strong requirements on optical homogeneity (average refractive index fluctuations and gradients) of the CBG.
What is needed is an ability to extend the prior art technology to the one of chirped Bragg gratings in PTR glass by providing an opportunity to perform the temporal, spatial, and spectral pulse shaping. It is important to note that all previous publications considered fiber and volume Bragg gratings operating in a retroreflecting mode. This is the simplest solution but it does not provide an opportunity for effective temporal, spatial and spectral control of processed laser pulses. The single consideration of an inclined volume Bragg grating is provided in FIG. 4 the '049 patent. However, this consideration is not correct because this inclined grating is not reciprocal and, therefore, different spectral components of a laser pulse incurs different lateral walk off (it is described above) and spatial profile of a laser pulse will be dramatically distorted.
The present invention is based on a new type of an optical element which is a multipass reciprocal inclined chirped volume Bragg grating. This invention provides a practical solution for stretching of ultrashort laser pulses to very long durations and recompression of these pulses without distortions by means of a new type of optical device which comprises one or several multipass reciprocal inclined chirped volume Bragg gratings with spatially variable parameters: period, direction of grating vector, refractive index modulation, and average refractive index combined with phase and amplitude masks.